An enquiry for a three-screw pump often begins with two figures, such as 100 L/min and 35 bar. Both are necessary, but they are not enough to select the pump. The same flow and pressure can require a different screw size, speed, bearing arrangement and motor when the liquid changes from light diesel to cold heavy oil.
Viscosity is part of the answer. So are lubricity, operating temperature, suction conditions and cleanliness. A three-screw pump works very well inside the right operating window, but it is less forgiving when the liquid does not suit its internal design.
MISLIER’s SN and T3S ranges are three-screw pumps intended mainly for clean, lubricating or partly lubricating liquids. They should not be confused with progressive cavity pumps for sludge or hygienic twin-screw pumps designed for products containing soft solids.
Why Three Screws Produce Smooth Flow
Inside the pump are one driven main screw and two idler screws. The profiles mesh within a close-fitting liner and form a series of sealed cavities. As the screws rotate, these cavities move axially from suction to discharge, carrying the liquid with them.
Many small volumes are moving at the same time, so delivery is continuous and pressure pulsation is low. This is useful in lubrication, fuel supply and hydraulic systems where a stable flow is more important than repeatedly accelerating and stopping the liquid.
The pump is a rotary positive displacement pump. It displaces volume, while the downstream resistance determines the pressure required from the pump. If a discharge valve is closed, the pump continues trying to move liquid and pressure can rise quickly.
For this reason, a three-screw pump system needs suitable overpressure protection. An internal relief valve can protect the pump during a short pressure event, but it is not a normal control valve and should not be used as a continuous bypass. Recirculating liquid through the internal valve generates heat and places unnecessary load on the pump.
The pumped liquid also provides lubrication inside the pumping element. The screws operate with close clearances and depend on an adequate fluid film. Dry running, abrasive particles and unsuitable low-lubricity liquids can damage the internal surfaces. Self-priming capability does not remove the need to fill, vent and protect the pump correctly during startup.
Let the Liquid Decide Whether the Pump Is Suitable
Three-screw pumps are commonly used for lubricating oil circulation, hydraulic oil, diesel, heavy fuel oil, burner supply and marine fuel or lubrication duties. Their compact construction and smooth delivery are well suited to these clean oil services.
Some vegetable oils, emulsions, water-glycol mixtures and viscous industrial liquids may also be considered, but only after their actual properties have been checked. The name of the industry is not enough. A lubricating vegetable oil and a thin cleaning chemical may both be used in a food plant, yet only one may be a natural fit for a standard three-screw pump.
The first question is whether the liquid contains solid particles. MISLIER’s standard selection range requires the medium to be free of solids. Sand, fibres, scale and abrasive contamination can score the screws and liner. A liquid that needs a progressive cavity or twin-screw pump should not be forced into a three-screw selection simply because all three products contain the word “screw”.
The next question is lubricity. Good lubricity supports internal bearing and screw surfaces. When lubricity is lower, the catalogue recommends an external bearing arrangement so that the bearing is not dependent on the pumped liquid in the same way. Plain water or a water-like chemical should therefore be reviewed carefully rather than treated as a routine application.
Viscosity must be stated at the pumping temperature. Heavy oil may have one viscosity after heating and a much higher value during cold start. The cold condition increases suction loss and starting torque. A pump that reaches the required flow after the liquid is warm can still be difficult to start if the pipe, filter and motor were selected only for normal operation.
Temperature also affects the seal and material choice. Corrosiveness, additives and leakage requirements must be considered with the body, liner and shaft-seal arrangement. A catalogue list of possible liquids is a starting point, not a universal compatibility approval.
Use Flow, Pressure, Viscosity and Speed Together
The overall catalogue envelope for the MISLIER SN and T3S families is 10 to 3600 L/min, with working pressure up to 4.0 MPa. The listed viscosity range is 3 to 750 mm²/s, operating speed is 500 to 3000 r/min and liquid temperature is -5 to 150°C. Allowable suction lift is shown as up to 8 m.
These figures describe a complete product family. They are not one operating point. A model selected near the high-flow end does not automatically deliver the maximum pressure at the highest viscosity and temperature. The individual performance table must be checked for screw size, helix angle, speed, slip and shaft power.
The MISLIER guide gives a useful starting rule for speed. When viscosity is at or below 380 mm²/s, 1450 or 2900 r/min may be considered, with 1450 r/min recommended as the normal option. When viscosity is above 380 mm²/s, the guide recommends 950 r/min. Lower speed helps reduce inlet velocity and can improve suction conditions for viscous liquid.
Motor selection depends on more than rated flow. Differential pressure and viscosity both influence shaft power. The motor must also have enough starting torque for the coldest expected liquid. Choosing the motor from the warm operating condition alone can leave too little margin at startup.
Suction conditions require a separate NPSH check. NPSH available from the tank and pipework must be greater than the pump’s NPSH required at the selected duty. Long suction pipe, small diameter, cold oil, high speed and a dirty filter all reduce the available margin. The catalogue’s suction-lift limit cannot replace this calculation.
Filtration is necessary because of the pump’s close internal clearances. The MISLIER guide specifies a 40 to 80 mesh filter before the inlet. The filter area should be selected for the viscosity, not only for the connection diameter. A filter that works normally with warm oil can create a large pressure drop when the same oil is cold.
Flow control also needs care. Throttling the discharge of a positive displacement pump raises pressure rather than reducing flow in the same way as a centrifugal pump. If variable flow is required, speed control or an appropriately designed bypass arrangement should be evaluated with the system and motor.
Choosing Between the MISLIER SN and T3S Series
SN and T3S share the same basic three-screw operating principle, but their configuration options are organised differently.
| Selection point | SN series | T3S series |
| General construction | Conventional configurable range | Compact integrated pump-and-liner design |
| Installation | Side-inlet horizontal, rear-inlet horizontal, vertical, bracket horizontal and immersion | Horizontal, vertical and immersion, including versions with or without mounting plate |
| Product length | Determined by selected construction | Short and long versions available |
| Port arrangement | Several inlet and outlet directions through model coding | Multiple inlet and outlet directions, useful for compact packages |
| Bearing choice | Internal or external according to medium and model code | Internal or external bearing option |
| Seal choice | Mechanical seal or oil-seal configuration according to model | Lip-type oil seal or mechanical seal |
| Material coding | Grey iron, welded steel or cast steel body options with several liner choices | Material and seal suitability confirmed for the specific service |
SN is useful where the pump must match an existing pipe arrangement or a specified combination of installation, body and liner materials. T3S is attractive for compact packaged systems, flexible port orientation and immersion installations. Neither series should be selected from its name alone. The performance table and complete model code still need to match the liquid and duty.
Before requesting a final three-screw pump selection, provide:
- exact liquid name and composition;
- viscosity at startup and normal temperature;
- operating temperature and density;
- lubricity, corrosiveness and solids content;
- required flow and differential pressure;
- suction pressure or tank level relative to the pump;
- installation direction and required port orientation;
- material, seal and leakage requirements;
- voltage, frequency and operating schedule.
A three-screw pump is a strong choice when the liquid is clean, sufficiently lubricating and able to reach the inlet without excessive loss. Its smooth flow and compact design then become real operating advantages. If solids, very low lubricity or poor suction conditions fall outside the pump’s working window, changing the pump type is usually more reliable than trying to solve every problem by increasing motor power or selecting a larger model.